Some facts:
The Songa Mercur is currently drilling in Block 05-1c in Vietnamese waters.
The reported location in the letter is within that Block - I don't have access to the exact coordinates of the well.
The distance to the last reported location is 306 nm on a true bearing of 276.5 degrees:
Note that the bearing line runs just south of the Con Son Islands and across the tip of the Ca Mau peninsula.
For an aircraft at FL350 to be visible from the rig, it would need to be closer than 230 nm. At FL200, 175 nm.